It is well known to assay components of a material by measuring absorption coefficients of the material for light at suitable wavelengths. A component of a material contributes to an absorption coefficient of the material for light at a given wavelength in proportion to σρ where ρ is a concentration of the component and σ is an absorption cross-section of the component for light at the given wavelength.
To determine an absorption coefficient of a material for light at a given wavelength, generally, a sample of the material is illuminated with light at the given wavelength. An amount of the light that is transmitted through the sample is measured to determine attenuation that the light suffers in passing through the sample and the Beer-Lambert law is then used to determine the absorption coefficient. If α represents the absorption coefficient and L the optical path-length of the light through the material, then by the Beer-Lambert law I=Ioexp(−αL), where I is the intensity of light transmitted through the material and Io is the intensity of light incident on the material.
Generally α is a function of concentration of a plurality of different components in the material. In order to determine concentration of a particular component of the material, α is measured at a plurality of different wavelengths. The concentration of a particular component of the material is determined from known absorption cross-sections of components of the material and the measurements of the absorption coefficient at the different wavelengths. U.S. Pat. No. 5,452,716 to V. Clift, the disclosure of which is incorporated herein by reference, describes measuring absorption coefficients for blood at a plurality of wavelengths to assay blood glucose.
Numerous devices, hereinafter referred to as “photometers”, of various designs are available for measuring an absorption coefficient of a material. The devices comprise a suitable light source, such as a laser or LED, which provides a beam of light that is passed through a sample of a material to be tested. Intensity of the provided beam of light is measured to provide a value of Io and a measurement of intensity of light transmitted through the sample provides a value for I. A value for an optical path-length L of the beam of light through the sample is generally determined from a shape of the sample, or in the case of a liquid, often from a shape of a cuvette that contains the liquid.
In some photometers, referred to as “vertical-beam photometers”, that are used to determine absorption coefficients of a liquid, a sample of the liquid is held in an open receptacle. A beam of light is transmitted “vertically” through the open end of the receptacle, the liquid contained in the receptacle and the bottom of the receptacle to determine attenuation of the beam and thereby an absorption coefficient of the liquid. The optical path-length L of the light beam through the liquid is determined by the height to which the receptacle is filled with the liquid and a shape of a meniscus formed at an interface of the liquid with the air.
In general, both the height of a liquid sample in a receptacle of a vertical-beam photometer and the shape of its meniscus cannot be controlled to an accuracy with which dimensions of a cuvette can be controlled. As a result, optical path-lengths of light through liquid samples in a vertical-beam photometer are generally not as accurately known or controllable as optical path-lengths through samples in a photometer for which optical path-lengths are determined by dimensions of a cuvette. Measurements of absorption coefficients provided by vertical-beam photometers are therefore generally not as accurate as measurements of absorption coefficients provided by other types of photometers.
However, vertical-beam photometers are popular because they enable rapid sampling of large numbers of liquid samples. The receptacles that hold liquid samples to be tested are generally formed as small wells in “trays” produced from a suitable material. The wells in a tray are easily and quickly filled with liquids to be tested. Once filled, the tray is rapidly positioned to expose the liquid in each of the wells to a beam of light that the photometer provides for measuring absorption coefficients.
U.S. Pat. No. 6,188,476, the disclosure of which is incorporated herein by reference, discusses the problem of determining optical path-lengths of liquid samples whose absorption coefficients are measured using vertical beam photometry. The patent describes methods for determining optical path-lengths of the sample solutions using calibration measurements of path-lengths at two different wavelengths for various common solvents that the liquid samples might contain.
In addition to errors in absorption coefficient measurements generated by errors in determination of optical path-lengths L, absorption coefficient measurements provided by photometers are often subject to error resulting from variations in intensity of light Io provided by the light source and drift in sensitivity of a detector used to determine I.
In medical applications, a “medical” photometer is used to measure absorption of light by a patient's blood at various wavelengths, optionally to provide an assay of a component, such as glucose, in the blood. Often the medical photometer is configured to draw blood from the patient or to receive blood from a system, such as for example, a heart lung machine, through which the patient's blood is flowing and shunt the blood through a flow cell. The medical photometer illuminates the blood in the flow cell with light at a suitable wavelength or wavelengths and determines how much of the light is transmitted through the flow cell to acquire absorption measurements for the blood at the wavelength or wavelengths.
However, for many medical purposes, it is desired to acquire absorption measurements in blood for light at the mid infra-red wavelength range. For these wavelengths of light, blood is a relatively strong optical absorber and light transmitted into a flow cell through which blood is flowing is strongly attenuated with distance that the light propagates in the blood. In order for a sufficient amount of light to pass through the flow cell so that reliable absorption measurements for the blood can be acquired, the flow cell must generally be made relatively small in a region through which the light is transmitted to acquire the absorption measurements. Often, a flow cell-cross-section at a location at which absorption measurements of blood are made is so small that the path length of light through the blood at the location is only about 20 microns in length. For flow cells comprising a region having a cross-section with such a small dimension, blood has a tendency to clot in the region and not only block the flow cell but generate a possible threat to a patient's health.